The masterflats are 1024x1024 FITS images (gzipped) named according to the
optical configuration (wf or hr camera and filter name or number), the array
used, and the dates they were taken and whether they are produced from 
twilight sky flats (sf) or from dome lamps (df). 

In all cases each masterflat is the result of 16-20 individual bright and 
faint flats (dithered by 10" in the case of twilight flats) resulting in 
8-10 differential flats which are median combined (with additive scaling) 
to filter out stars (in the case of twilight flats) and improve signal to 
noise. All flats are normalized. 

The file name "pa14_sf_wf_ks_sci" means this is a science grade array 
masterflat for the WF camera and Ks filter obtained on the 14/1-2006 
from a set of twilight sky flats. See our filter web page 
www.not.iac.es/instruments/notcam/filters for the numbering convention.
The first four letters of the name are the standard file dating
convention at the NOT: the first gives the year, the second the month 
and the last two give the day (starting date for a night). 
Domeflats (df), twilight sky flats (sf), WF camera (wf) and HR camera (hr).

Anlaug Amanda Djupvik. 01.09.2006.
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Notes:

- Images taken after 01.01.2006 are flipped in x-axis with respect to the 
older images. This was done when introducing the Multi-Extension Fits 
format (MEF) and using the NOT2MEF translator to get fits headers up to date.

- Since 2007 the raw flats are badpixel corrected first.

- The flats listed from Dec-2007 ("ql13" and onwards, i.e. flats obtained 
with the new science array) have been corrected for the dc-gradient that 
arises in the differential flats and otherwise leaves horizontal stripes. 

- In old flats the combined image was smoothed with a 3x3 pixel box to get 
rid of individual bad pixels and dead columns. Since January 2009 this has
been avoided by correcting zero-valued pixels by interpolation. For some
2009 master flats both the old and new versions are available. 

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